The objective of the present proposal is to elucidate the electronic mechanisms of interactions between DNA and skin-sensitizing and skin-carcinogenic coumarins under the influence of light. Information concerning the mode of interactions, identities, and stereochemistry of photoproducts is to be obtained; and finally, biological effects of the photocycloaddition reaction of coumarins to DNA of Bacillus subtilis are to be investigated in an attempt to elucidate molecular mechanisms of coumarins in skin-sensitization in general and skin-cancer in particular. In addition, synthesis and photochemical reactions of a reactive model skin-sensitizer, benzodipyrone (coumarylpyrone), with DNA via cross-linking of double strands being carried out as a new probe for monitoring molecular mechanisms involved in skin-sensitization, photomutagenesis, and photo-induced carcinogenesis.